THE CRACK-TIP MECHANICS AND GROWTH-RATES OF SMALL FATIGUE CRACKS IN ASTROLOY

The micromechanics of naturally initiated small fatigue cracks in coar se- and fine-grained nickel-based ASTROLOY have been studied at ambien t and elevated temperature using a cyclic loading stage for the scanni ng electron microscope (SEM) and the stereoimaging technique. The obje ctive of these experiments was to obtain a fracture mechanics descript ion of crack driving force for use in engineering damage tolerance ass essments. Cracks were initiated from small pores and slip lines within grains at ambient temperature in bending but were grown in tension at 20 degrees C and 600 degrees C. When expressed in terms of the linear elastic stress intensity factor range, Delta K, small cracks grew fas ter and at lower Delta K than did large cracks, as has been found for many other materials. Displacements were measured around the tips of s mall cracks, from which both crack opening displacements (CODs), and s trains were derived. Mixed mode CODs were typically found. Because the small cracks exhibited large levels of crack-tip plasticity, an equiv alent stress intensity factor (a local crack driving force based on De lta J) was computed using measured crack-tip parameters. Crack opening load, measured by direct observation of the crack peeling open under high resolution conditions, was used to calculate an effective stress intensity factor. The relationships between the applied Delta K, as co mputed by standard fracture mechanics methods, the equivalent stress i ntensity factor, and the effective stress intensity factor for small c racks were found to be different than for large cracks. Crack growth r ates for small and large fatigue cracks were correlated through the us e of equivalent stress intensity factors.